Liquid crystal display device

ABSTRACT

A liquid crystal display device having a pair of substrates with a liquid crystal layer interposed between the pair of substrates. One of the pair of substrates includes a plurality of scanning electrodes, a plurality of signal electrodes arranged so as to cross the plurality of scanning electrodes, a plurality of thin film transistors arranged in the vicinity of crossing points of the scanning electrodes and the signal electrodes, a plurality of common electrodes, and a plurality of pixel electrodes arranged between each of the common electrodes. An electric field is formed in the liquid crystal layer by applying a voltage to the pixel electrodes and the plurality of common electrodes. The other substrate of the pair of substrates includes color filters, an insulating film for flattening the color filters arranged on the color filters, and an orientation control film arranged on the insulating film.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of U.S. application Ser. No. 09/543,337,filed Apr. 5, 2000, which is a continuation of U.S. application Ser. No.08/908,184, filed Aug. 7, 1997, now U.S. Pat. No. 6,124,915, which is acontinuation of U.S. application Ser. No. 08/744,451, filed Nov. 6,1996, now U.S. Pat. No. 5,737,051, which is a continuation of U.S.application Ser. No. 08/123,472, filed Sep. 20, 1993, now U.S. Pat. No.5,598,285, the subject matter of which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a liquid crystal display devicehaving at least one, and preferably a plurality of pixel elements.

[0003] In a standard liquid crystal display device, the pixel element isformed in a liquid crystal layer (normally common to the pixel elements)extending in a plane, and there is at least one polarizing arrangementparallel to the plane of the liquid crystal layer. By applyingelectrical signals to the liquid crystal layer using suitableelectrodes, it is possible to vary the angle of polarization ofpolarized light passing through the liquid crystal layer, and thereby tovary the optical transmissivity of a liquid crystal display device withthe change in polarization relative to the polarizing arrangement.Normally, in such a liquid crystal display device, the polarizingarrangement is formed by two polarizing plates, one on each side of theliquid crystal layer, but it is also possible to provide an arrangementwith a single polarizing plate on one side of the liquid crystal layerand a reflective element on the other side of the liquid crystalelement.

[0004] In standard liquid crystal display devices, electrical fields aregenerated by electrodes arranged perpendicular to the plane of theliquid crystal layer. Therefore, if the change in the liquid crystallayer due to the electric fields is to be visible, the size of thoseelectrodes needs to be large; and, therefore, it is necessary to usetransparent electrodes. Furthermore, at least two layers are normallyneeded between the transparent electrodes on each side of the liquidcrystal layer and the liquid crystal layer itself. One such layer formsan orientation layer for the liquid crystal layer, but a furtherinsulating layer is then needed between the orientation layer and thetransparent electrode.

[0005] In International Patent Application No. PCT WO91/10936, a liquidcrystal display device was disclosed in which electrical signals wereapplied to the liquid crystal layer so as to generate electric fieldshaving components in a direction parallel to the plane of the liquidcrystal layer. Such parallel field components cause reorientation of themolecules of the liquid crystal layer, thereby varying the opticaltransmissivity of the liquid crystal display device.

[0006] In PCT W091/10936, it was proposed that the electrodes forapplying such field were, for each pixel element, in the form of combs,the teeth of the comb formed by one electrode extending into the spacesbetween the teeth of the comb formed by the other electrode. The teethof each electrode were electrically connected in common, and a voltagewas applied between the electrodes.

[0007] JP-B-63-21907 (1988) also disclosed a liquid crystal displaydevice in which electrical signals were applied to the liquid crystallayer so as to generate electric fields having components in a directionparallel to the plane of the liquid crystal layer. As in PCT W091/10936,the electrodes for applying such fields were, for each pixel element, inthe form of combs. Use of comb-shaped electrodes was also disclosed inU.S. Pat. No. 4,345,249.

[0008] In each of these known arrangements, each pixel element has firstand second electrodes of comb shape, with the teeth of one combextending between the teeth of the other comb. Voltages are then appliedto the electrodes by a suitable control circuit. It is important to notethat the teeth of the comb-shaped electrodes are not electricallyindependent, so that the size of the pixel is determined by the size ofthe comb-shaped electrode.

[0009] The principles of operation of such devices, with comb shapedelectrodes, is also discussed in an article entitled “Field Effects InNematic Liquid Crystals Obtained With Interdigital Electrodes” by R. A.Soref in the Journal of Applied Physics, pages 5466 to 5468, vol. 45,no. 12 (December 1974), and in an article entitled “InterdigitalTwisted-Nematic-Displays” by R. A. Soref, published in the Proceedingsof the IEEE, pages 1710 to 1711 (December 1974).

SUMMARY OF THE PRESENT INVENTION

[0010] In the standard liquid crystal display devices discussed above,it is necessary to use transparent electrodes, which are formed onfacing surfaces of two substrates. However, in order to form suchtransparent electrodes, it is necessary to use a vacuum manufacturingoperation, such as sputtering, and thus the cost of manufacture of suchstandard liquid crystal display devices is high. Furthermore, it hasbeen found that such transparent electrodes have vertical geometricalirregularities, of the order of several tens of nanameters, whichprevents precise manufacture of active devices, such as thin filmtransistors needed to control the signals to the electrodes. Also, ithas been found that parts of such transparent electrodes may becomedetached, to cause point or line defects. Thus, it has proved difficultto manufacture liquid crystal devices both reliably and cheaply.

[0011] Such conventional liquid crystal display devices also havedisadvantages in terms of picture quality. The problem of verticalgeometrical irregularities in the transparent electrodes has beenmentioned above, but similar irregularities around the controllingtransistors may result in orientation failure domains being formed,requiring a light shielding film to cover such transistor devices,reducing the light utilization efficiency of the liquid crystal device.Also, such conventional liquid crystal display devices have thedisadvantage that there is a significant change in brightness when thevisual angle is changed, and reversion of some gradation levels canoccur in a half-term display, at some view angles.

[0012] Although the use of comb shaped electrodes, such as previouslydiscussed, involves the need for transparent electrodes, furtherproblems have been found. While the use of such comb-shaped electrodesoffers theoretical advantages, those are limited by practicalconsideration which have to be taken into account when the comb-shapedelectrodes are used. If the teeth of such comb-shaped electrodes have awidth of 1 to 2 micrometers, satisfactory practical operation can beachieved. However, it is extremely difficult to form such fine teethover a large substrate without defects. Thus, in practice, the aperturefactor of the liquid crystal display device is reduced, because of theneed to provide relatively wide electrode teeth. There is thus atrade-off between aperture factor and production yield, which isundesirable.

[0013] Therefore, the present invention seeks to provide a liquidcrystal display device which is more suitable for mass production thanthe known liquid crystal display devices discussed above. The presentinvention has several aspects.

[0014] In the liquid crystal display device according to the presentinvention there are features which are common to all the aspects. Thedevice has a liquid crystal layer, and at least one polarizingarrangement, which is normally provided in the form of a pair ofpolarizing plates disposed on opposite sides of the liquid crystallayer. The device has at least one, and normally a plurality, of pixelelements and there are electrodes which receive electrical signals forcontrolling the optical transmissivity of light through the device. Asin e.g. JP-B-63-21907 (1988) discussed above, the electrical signals areapplied such that electrical fields are generated in the liquid crystallayer with components parallel to the plane of the liquid crystal layer.The various aspects of the present invention, which will be discussedbelow, then relate to the electrode arrangement of a pixel element(s)and also to the materials and optical arrangements of the materials ofthe liquid crystal display device.

[0015] In a first aspect of the present invention, each pixel elementhas a pixel electrode extending in a first direction within the pixel,and there are also signal wiring electrodes extending in the samedirection over several of the pixel elements. There are also commonelectrodes extending in that first direction over more than one of thepixel elements.

[0016] There may be a pair of pixel electrodes for each pixel element,with the signal wiring electrodes then extending between a pair of pixelelectrodes at each pixel element. There is then a pair of commonelectrodes, with the pair of pixel electrodes being disposedtherebetween, so that electrical fields are generated in oppositedirections for each pixel element.

[0017] Preferably, all the electrodes are on the same side of the liquidcrystal layer. Arrangements are also possible, however, in which thecommon electrodes are on the opposite side of the liquid crystal layerformed the other electrodes. In either case, if there is insulatingmaterial between the common electrode and the pixel electrode for eachpixel element, a capacitive device may be formed therebetween.

[0018] In practice, it is possible for the common electrodes to beprovided in common for two adjacent pixel elements, by interacting withpixel electrodes on opposite sides of each common electrode.

[0019] In a second aspect of the present invention, each pixel can beconsidered to have a elongate transistor element extending in a firstdirection, that elongate transistor element having at least one elongateelectrode. There is also at least one elongate common electrodeextending in the same direction as the elongate transistor element. Inthe second aspect, an insulating film separates the at least oneelongate electrode of the elongate transistor element and the at leastone common electrode.

[0020] In a third aspect of the present invention, each pixel has againan elongate transistor element extending in one direction, and at leastone elongate common electrode extending in the same direction. Theelongate transistor element has at least one elongate electrode, andthere is an insulating film extending over and in direct contact withthat at least one elongate electrode. That insulating film is also indirect contact with the liquid crystal layer. Preferably, thatinsulating film is an organic polymer.

[0021] In a fourth aspect of the present invention, each pixel can beconsidered to have a transistor element with a pair of first electrodes(pixel electrodes), a signal electrode between the first electrodes, anda gate electrode. The first and second electrodes extend in a commondirection, as do a pair of common electrodes. A transistor element isthen (in plan) disposed between the pair of common electrodes. As haspreviously been mentioned, the common electrodes and the transistorelement may be on the same side of the liquid crystal layer, or they maybe on opposite sides.

[0022] In a fifth aspect, for any one pixel element, each one of saidpairs of common electrodes thereof forms a corresponding one of saidpair of common electrodes for pixel elements adjacent to said any onepixel element.

[0023] The five aspects of the present invention discussed above allrelate to an electrode arrangement of the liquid crystal display device.The aspects of the invention to be discussed below relate to the opticalarrangement and materials of the liquid crystal display device.

[0024] In a sixth aspect of the present invention, the angles betweencomponents of electric fields in a direction parallel to the plane ofsaid liquid crystal layer and the direction of orientation of moleculesat opposite surfaces of the liquid crystal layer are the same, and theproduct of the thickness of the liquid crystal layer and the refractiveindex anisotropy of the liquid crystal layer is between 0.21 μm and 0.36μm.

[0025] In a seventh aspect of the present invention, the absolute valueof the difference between the angles between components of electricfields in a direction parallel to the plane of said liquid crystal layerand the direction of orientation of molecules at opposite surfaces ofthe liquid crystal layer is not less than 80° and not greater than 100°,and the product of the thickness of the liquid crystal layer and therefractive index anisotropy of the liquid crystal layer is between 0.4μm and 0.6 μm.

[0026] In an eighth aspect of the present invention, the dielectricconstant anisotropy of the liquid crystal layer is positive, and theabsolute value of the angle between components of the electric fields ina direction parallel to the plane of said liquid crystal layer and thedirection of orientation of molecules at the surface of the liquidcrystal layer is less than 90°, but not less than 45°.

[0027] In a ninth aspect of the present invention, the dielectricconstant anisotropy of the liquid crystal layer is negative, and theabsolute value of the angle between components of the electric fields ina direction parallel to the plane of said liquid crystal layer and thedirection of orientation of molecules at the surface of the liquidcrystal layer is greater than 0°, but not greater than 45°.

[0028] In a tenth aspect of the present invention, the dielectricconstant anisotropy of the liquid crystal layer is positive, and thevalue of the difference between: i) the angle between components of theelectric fields in a direction parallel to the plane of said liquidcrystal layer and the direction of orientation of molecules at thesurface of the liquid crystal layer; and ii) the angle of thepolarization axis of said at least one polarizing plate and saidcomponents of the electric fields in a direction parallel to the planeof said liquid crystal layer, is 3° to 15°.

[0029] In an eleventh aspect of the present invention, the dielectricconstant anisotropy of the liquid crystal layer is negative, and thevalue of the difference between i) the angle of the polarization axis ofsaid at least one polarizing plate and said components of the electricfields in a direction parallel to the plane of said liquid crystallayer, and ii) the angle between components of the electric fields in adirection parallel to the plane of said liquid crystal layer and thedirection of orientation of molecules at the surface of the liquidcrystal layer, is 3° to 15°.

[0030] In a twelfth aspect of the present invention, the direction oforientation of molecules of said liquid crystal layer at a surface ofsaid liquid crystal layer parallel to the plane of said liquid crystallayer and said surface is not more than 4°.

[0031] Although various aspects of the present invention have beendiscussed above, a liquid crystal display device embodying the presentinvention may incorporate combinations of such aspects. Depending on theresulting combination, the present invention provides advantages in themanufacture and/or operation of a liquid crystal display device, andthese advantages will be discussed in more detail later.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the present invention will now be described indetail, by way of example, with reference to the accompanying drawings,in which:

[0033] FIGS. 1(a) to 1(d) are schematic diagrams illustrating thebehavior of liquid crystal molecules in a liquid crystal display deviceaccording to the present invention;

[0034] FIGS. 2(a) and 2(b) are plan and sectional views, respectively,of an embodiment, showing the structure of a thin film transistor, whichmay be used in the present invention;

[0035] FIGS. 3(a) and 3(b) are graphs illustrating electro-opticalcharacteristics (visual angle dependence),

[0036]FIG. 3(a) corresponding to the present invention and

[0037]FIG. 3(b) corresponding to a comparative example;

[0038]FIG. 4 shows an embodiment of the present invertion in which thepixel electrode (source electrode), the common electrode, the scanningelectrode, and the signal electrode (drain electrode) are all arrangedon only one of the substrates of the device;

[0039] FIGS. 5(a) and 5(b) are plan and sectional views, respectively,of an embodiment of the present invention in which the pixel electrode(source electrode) and the signal electrode (drain electrode) arearranged at the center of the pixel to divide the pixel into two parts;

[0040]FIG. 6 is a diagram illustrating the angles formed by the longaxis orientation direction of the liquid crystal molecules at theinterface of the polarizing axis of the polarizing plate, and the phaseadvance axis of the phase difference plate, with respect to thedirection of the electric field;

[0041]FIG. 7 is a graph illustrating the electro-optical characteristicsof various embodiments with different orientation directions of the longaxis of the liquid crystal molecules at the interface when thedielectric constant anisotropy is positive;

[0042]FIG. 8 illustrates schematically a liquid crystal display drivingsystem which may be used in the present invention;

[0043]FIG. 9 illustrates an example of the present invention applied toa liquid crystal display transmission type optical system;

[0044]FIG. 10 illustrates an example of the present invention applied toa liquid crystal display reflection type optical system;

[0045]FIG. 11 is a graph illustrating the electro-opticalcharacteristics of various embodiments with different orientationdirections of the long axis of the liquid crystal molecules at theinterface when the dielectric constant anisotropy is negative;

[0046] FIGS. 12(a) and 12(b) are graphs illustrating the electro-opticalcharacteristics of various embodiments with different thicknesses d ofthe liquid crystal layer when the dielectric constant anisotropy isnegative;

[0047] FIGS. 13(a) and 13(b) are graphs illustrating the electro-opticalcharacteristics of embodiments in which polarizing plates are arrangedso that the dark state can be obtained by applying a small non-zeroelectric field;

[0048]FIG. 14 is a graph illustrating the characteristics of a normallyopen type device and the characteristics when its residual phasedifference at the interface is compensated;

[0049]FIG. 15(a) is a plan view and FIGS. 15(b) and 15(c) are sectionalviews of a fourth embodiment in which a capacitive device is formedbetween a common electrode and the pixel electrode on the respectivefacing interfaces of the substrates;

[0050]FIG. 16 is a schematic plan view of a pixel when no electric fieldis applied, for a fifth embodiment of the present invention in which thepixel electrode and the common electrode are arranged at differentlayers separated by an insulating layer;

[0051]FIG. 17 is a schematic cross sectional view of a pixel when noelectric field is applied in the fifth embodiment in which the pixelelectrode and the common electrode are arranged at different layersseparated by an insulating layer;

[0052]FIG. 18 is a schematic plan view of a pixel when no electric fieldis applied in a sixth embodiment of the present invention in which thepixel electrode and the common electrode are arranged at differentlayers separated by an insulating layer, the pixel electrode being aclosed loop and the common electrode being cruciform;

[0053]FIG. 19 is a schematic plan view of a pixel when no electric fieldis applied in a seventh embodiment of the present invention in which thepixel electrode and the common electrode are arranged at differentlayers separated by an insulating layer, the pixel electrode having theshape of the letter I and the common electrode being a closed loop;

[0054]FIG. 20 is a schematic cross sectional view of part of a pixelwhen no electric field is applied in an eighth embodiment of the presentinvention in which the pixel electrode and the common electrode arearranged at different layers separated by an insulating layer, and thereis an insulating layer between the scanning electrode and the commonelectrode;

[0055]FIG. 21 is a schematic cross sectional view of part of a pixelwhen no electric field is applied in a ninth embodiment of the presentinvention in which the pixel electrode and the common electrode arearranged at different layers separated by an insulating layer, and thecommon electrode is formed on the protection insulating film; and

[0056]FIG. 22 is a schematic cross sectional view of part of a pixelwhen no electric field is applied in a tenth embodiment of the presentinvention in which the pixel electrode and the common electrode arearranged at different layers separated by an insulating layer, and boththe scanning electrode and the common electrode are made from aluminumcoated with self-oxidized film.

DETAILED DESCRIPTION

[0057] Before describing embodiments of the present invention, thegeneral principles underlying the present invention will be explained.

[0058] First, suppose that the angle of a polarized light transmissionaxis of the polarized plate with respect to the direction of theelectric field is φ_(P), the angle of the longitudinal axis (opticalaxis) of liquid crystal molecules near the interface with respect to theelectric field is φ_(LC), and the angle of the phase advance axis of thephase difference plate inserted between the pair of polarizing plateswith respect to the electric field is φ_(R) as shown in FIG. 6. Becausethere are normally pairs of polarizing plates and liquid crystalinterfaces, i.e. upper and lower plates and interfaces, these arerepresented as φ_(P1), φ_(P2), φ_(LC1) and φ_(LC2). FIG. 6 correspondsto a top view of FIG. 1, to be described later.

[0059] Next, the operation of embodiments of the present invention willbe described with reference to FIGS. 1(a) to 1(d).

[0060] FIGS. 1(a) and 1(b) are side cross sections showing the operationof a liquid crystal display device according to the present invention.FIGS. 1(c) and 1(d) are front views of the liquid crystal displaydevice. In FIGS. 1(a) to 1(d), the thin film transistor device isomitted. In this invention, stripes of electrodes are used to form aplurality of pixels, but FIGS. 1(a) to 1(d) show the structure of onlyone pixel (one cell). The side cross section of the cell when no voltageis applied is shown in FIG. 1(a), and the top view is shown in FIG.1(c).

[0061] Linear electrodes 1, 2, are formed on the inner sides of pairedtransparent substrates 3, over which an orientation control film 4 isapplied and subjected to an orientation processing. A liquid crystal isheld between the paired substrates 3. Bar shaped liquid crystalmolecules 5 of the liquid crystal are oriented so that they are at aslight angle to the longitudinal direction of the electrodes 1, 2, i.e.45 degrees≦φ_(LC)<90 degrees, when no electric field is applied. Thisdiscussion assumes that the orientation directions of the liquid crystalmolecules on the upper and lower interfaces are parallel to each other,i.e. φ_(LCL)=φ_(LC2). It is also assumed that the dielectric constantanisotropy of the liquid crystal is positive.

[0062] When an electric field 7 is applied, the liquid crystal molecules5 change their directions and are aligned with the direction of theelectric field 7, as shown in FIGS. 1(b) and 1(d). By arranging thepolarizing plates 6 at a specified angle 9, it is possible to change thelight transmission factor when the electric field is applied. Thus, itis possible to provide a display having a contrast without usingtransparent electrodes.

[0063] In FIG. 1(b), the angle between the substrate surface and thedirection of the electric field looks large and it seems not to beparallel to the substrates. In reality this is the result of magnifyingthe thickness direction in FIG. 1(b) to facilitate illustration, and theangle is actually less than 20 degrees.

[0064] In the following description, the electric fields having aninclination of less than 20 degrees are generally referred to as lateralelectric fields. FIGS. 1(a) and 1(b) show an arrangement in which theelectrodes 1, 2, are provided separately on the upper and lowersubstrates, respectively. However, it is possible to arrange them on onesubstrate and produce the same effect. Because the wiring pattern isvery fine and may therefore deform due to heat and/or external forces,arranging the electrodes on a single substrate is preferable because itpermits a more precise alignment.

[0065] Although the dielectric constant anisotropy of the liquid crystalis assumed to be positive, it may be negative. In that case, the initialorientation of the liquid crystal is set so that the liquid crystalmolecules have a slight angle |φ_(LC)| (i.e. 0 degree<|φ_(LC)|≦45degrees) with respect to a direction perpendicular to the longitudinaldirection of the electrodes 1, 2, (the direction of the electric field7).

[0066] Advantages which may thus be achieved with the present inventionwill now be explained.

[0067] (1) The First Advantage is That Enhanced Contrast may be AchievedWithout Using Transparent Electrodes.

[0068] There are two types of structure for producing contrast. The oneutilizes a mode in which the orientations of the liquid crystalmolecules 5 on the upper and lower substrates 3 are almost parallel toeach other (since color interface by birefringent phase difference isused, this mode is called a birefringent mode). The other structure hasa mode in which the orientations of the liquid crystal molecules S onthe upper and lower substrates 3 cross each other, twisting themolecular orientation in the cell (since light spiraling produced byrotation of the polarization plane in the liquid crystal compositionlayer is used, this mode is called a light spiraling mode).

[0069] In the birefringent mode, when a voltage is applied, themolecular long axis 8 (optical axis) changes its direction in a planealmost parallel to the substrate interface, changing its angle withrespect to the axis of a polarized plate (not shown in FIGS. 1(a) and1(b)) which is set at a specified angle. This results in a change in thelight transmission factor.

[0070] In the light spiraling mode, the application of a voltagesimilarly changes only the direction of the molecular long axis in thesame plane. This mode, however, utilizes a change in the light spiralingas the spiral is unraveled.

[0071] Next, a structure for making the display colorless and increasingthe contrast ratio will be explained below for two cases: one using thebirefringent mode and the other using the light spiraling mode.

[0072] II. Displaying in the birefringent mode: generally, when auniaxial birefringent medium is inserted between two orthogonalpolarizing plates, the light transmission factor, T/T₀, is expressed asfollows.

T/T ₀=sin²(2χ_(eff))·sin²(πd _(eff) ·Δn/λ)  Equation (1)

[0073] In Equation 1, χ_(eff) is the effective direction of the lightaxis of liquid crystal composition layer (an angle between the lightaxis and the polarized light transmission axis), d_(eff), is theeffective thickness of the liquid crystal composition layer havingbirefringence, Δn is the refractive index anisotropy, and λ is the wavelength of the light. In an actual cell the liquid crystal molecules arefixed at the interface and not all the liquid crystal molecules in thecell are parallel and uniformly oriented in one direction when anelectric field is applied. Instead, they are significantly deformedparticularly near the interface. It is therefore convenient to assume anapparent uniform state as the average of these states. In Equation 1, aneffective value is used for the light axis direction of the liquidcrystal composition layer.

[0074] To obtain a normally closed characteristic in which the displayappears dark when a low voltage V_(L) is applied and bright when a highvoltage V_(H) is applied, the polarizing plates should be arranged sothat the light transmission axis (or absorption axis) of one of thepolarizing plates is almost parallel to the orientation direction of theliquid crystal molecules (rubbing axis), i.e. φ_(P2)≈φ_(LC1)≈φ_(LC2).The light transmission axis of the other light polarizing plate isperpendicular to the first axis, i.e. φ_(P2)=φ_(P1)+90°. When noelectric field is applied, since χ_(eff) in Equation 1 is zero, thelight transmission factor T/T₀ is also zero. On the other hand, when anelectric field is applied, the value of χ_(off) increases in dependenceon the intensity of the electric field and becomes maximum at 45degrees. Then, if the light wavelength is assumed to be 0.555 μm, inorder to make the light colorless and the light transmission factormaximum, the effective value of d_(eff)·Δn should be set to half of thewavelength, i.e. 0.28 μm. Actually, however, there is a margin in thevalue, and values between 0.21 μm and 0.36 μm are suitable, with thevalues between 0.24 μm and 0.33 μm being preferred.

[0075] On the other hand, in order to obtain normally opencharacteristics in which the display appears bright when a low voltageV_(L) is applied and dark when a high voltage V_(H) is applied, thepolarizing plates should be arranged so that χ_(eff) in Equation 1 isalmost 45° when no electric field is applied or an electric field of lowintensity is applied. When an electric field is applied, the valueχ_(eff) decreases in dependence on the field intensity as opposed to thecase of the normally closed characteristic. However, because there is aresidual phase difference in the liquid crystal molecules fixed near theinterface even when χ_(eff) is minimum (i.e. zero), a significant amountof light will leak under this condition.

[0076] In an experiment conducted by the inventors of the presentinvention, in which the value of d·Δn was set between 0.27 and 0.37 andan effective voltage of 3 to 10 volts was applied, the residual phasedifference on the interface was 0.02 to 0.06 μm. Hence, by inserting aphase difference plate having a birefringence phase difference of about0.02 to 0.06 μm (this phase difference is represented as R_(f)) tocompensate for the interface residual phase difference, the dark statebecomes darker giving a high contrast ratio. The angle φ_(R), of thephase advance axis of the phase difference plate is set parallel to theeffective light axis χ_(eff) of the liquid crystal composition layerwhen the voltage is applied.

[0077] To make the dark state be as black as possible, the angle of thephase advance axis should be aligned precisely with the residual phasedifference that occurred when a voltage for displaying a dark state isapplied. Therefore, to make the dark state compatible with the increasedlevel of the transmission factor and the lightness of the bright state,the following relationship must be fulfilled

0.21μm<(d·Δn−R _(f))<0.36μm  Equation (2)

[0078] Or more preferably,

0.23μm<(d·Δn−R _(f))<0.33μm  Equation (3)

[0079] II. Displaying in the light spiraling mode: In a conventionaltwisted nematic (TN) system, when the value of d·Δn is set to around0.50 μm, a first minimum condition, a high transmission factor andcolorless light may be obtained. It has been found preferable to set thevalue in a range from 0.40 to 0.60 μm. The polarizing plates arearranged such that the transmission axis (or absorbing axis) of one ofthe polarizing plates is set almost parallel to the orientationdirection (rubbing axis) of the liquid crystal molecules on theinterface, i.e. φ_(LC1)≈φ_(LC2). To realize a normally closed typedevice, the transmission axis of the other polarizing plate is setparallel to the orientation direction of the liquid crystal molecules;and, for a normally open type, the transmission axis of the polarizingplate is set perpendicular to the orientation direction.

[0080] To eliminate light spiraling, it is necessary to set theorientation direction of the liquid crystal molecules near the upper andthe lower substrate interfaces so that they are almost parallel to eachother. If a 90° TN mode is assumed, the liquid crystal molecules on oneof the substrates must be turned nearly 90°. However, in displaying inthe birefringence mode, the liquid crystal molecules need only be turnedabout 45°. Furthermore, the birefringence mode has a lower thresholdvalue.

[0081] (2) The Second Advantage is That the Visual Angle Characteristicsmay be Improved.

[0082] In the display mode, the long axes of the liquid crystalmolecules are almost parallel to the substrate at all times and do notbecome perpendicular to the substrate, so that there is only a smallchange in brightness when the visual angle is changed. This display modeprovides a dark state, not by making the birefringence phase differencealmost zero by applying a voltage as in the case of a conventionaldisplay device, but by changing the angle between the long axes of theliquid crystal molecules and the axis (absorbing or transmission axis)of the polarizing plate. Thus, the display mode of the present inventiondiffers fundamentally from that of the conventional device.

[0083] In a conventional TN type of liquid crystal display device inwhich the long axes of the liquid crystal molecules are perpendicular tothe substrate interface, a birefringence phase difference of zero isobtained only in a visual direction perpendicular to the front or thesubstrate interface. Any inclination from this direction results in abirefringence phase difference, which means leakage of light in the caseof the normally open type, causing reduction of contrast and reversal oftone levels.

[0084] (3) The Third Advantage is That There is Improved Freedom in theSelection of the Materials of the Orientation Film and/or the LiquidCrystal, and so the Margin for the Related Process may thus beIncreased.

[0085] Since the liquid crystal molecules do not become erect, anorientation film for providing a large inclination angle (the anglebetween the long axis of the liquid crystal molecule and the interfaceof the substrate), which was used in a conventional device, is no longernecessary. In a conventional liquid crystal display device, when theinclination angle becomes insufficient, two states with differentinclination directions and domains bordering the two states occur,raising the possibility of a poor display. Instead of having such aninclination angle, the display system of the present invention may havethe long axis direction of the liquid crystal molecule (rubbingdirection) on the substrate interface set in a specified directiondifferent from 0° or 90° with respect to the electric field direction.

[0086] For example, when the dielectric constant anisotropy of theliquid crystal is negative, the angle between the electric fielddirection and the long axis direction of the liquid crystal molecule onthe substrate interface φ_(LC) (φ_(LC)>0) should exceed 0°, and normallyshould be more than 0.50°, preferably more than 2°. If the angle is tobe set at exactly 0°, two kinds of deformations with differentdirections, and domains of two different states and their bordering aregenerated, and a possibility of deterioration in display quality occurs.If the angle is set to more than 0.5°, the apparent long axis directionof the liquid crystal molecule (φ_(LC)(V)) increases uniformly withincreasing intensity of the electric field, and there is no possibilityof the long axis being inclined in the reverse direction, i.e.φ_(LC)(V)<0.

[0087] With this system, since no domains occur even if the angle(inclination angle) between the interface and the liquid crystalmolecule is small, it is possible to set the angle to have a smallvalue. The smaller the inclination angle, the greater will be theprocess margin for rubbing, improving the uniformity of the liquidcrystal molecule orientation. Hence, when the present process in whichan electric field is supplied in parallel to the interface combines witha low inclination technique, the orientation of the liquid crystalmolecule becomes more uniform, and display variations can be reducedmuch better than in a conventional system, even if there are variationsof the same magnitude in the manufacturing process.

[0088] Generally, there are fewer kinds of orientation films thatproduce a high inclination angle than those which produce a smallinclination angle. The present system increases the freedom in theselection of the orientation film material. For example, when an organicpolymer is used for the flattening film disposed over the color filtersand for the protective film disposed over the thin film transistors andis directly subjected to a surface orientation processing, such asrubbing, an organic film can be used with ease as the orientation filmsimultaneously because there is no need to provide an inclination angle.Hence, it becomes possible to simplify the process and to decrease thecost. In order to eliminate display irregularities due to variations inthe manufacturing process, the inclination angle preferably is set below4°, and more preferably below 2°.

[0089] Furthermore, freedom in the selection of the liquid crystalmaterial can be increased. as will be explained below.

[0090] In accordance with the present invention, the pixel electrodesand the common electrodes may have a structure in which an electricfield generally parallel to the interface of the substrate is applied tothe liquid crystal composition layer. The distance between theelectrodes can be chosen to be longer than the distance between themutually facing transparent electrodes of the conventional verticalelectric field active matrix type liquid crystal display device. Theequivalent cross sectional area of the electrode can be made smallerthan that of the conventional arrangement. Hence, the electricresistance between the paired pixel electrodes of the present inventioncan be significantly larger than that of the mutually facing transparentelectrodes of the conventional active matrix liquid crystal displaydevice.

[0091] Furthermore, the electrostatic capacitance between the pixelelectrode and the common electrode of the present invention can beconnected in parallel with capacitive devices, and the capacitive devicehaving a high electrical resistance can be achieved. Therefore, theelectrical charge accumulated in the pixel electrode can be held withease, and sufficient holding characteristics can be achieved even if thearea of the capacitive device is decreased. This area reduces theaperture factor and therefore needs to be small, if possible.

[0092] Conventionally the liquid crystal composition has an extremelyhigh specific resistance, for instance 10¹² Ωcm. However, in accordancewith the present invention, it is possible to use a liquid crystalcomposition having a lower specific resistance than the conventionalone, without causing any problems. That means not only increased freedomfor selection of the liquid crystal material, but also increases themargin for the processing. In other words, a defect in display qualityrarely occurs even if the liquid crystal is contaminated during theprocessing. Thus, the margin for the previously described variation atthe interface with the orientation film increases, and defects caused atthe interfaces are rare. Consequently, processes such as inspection andageing can be significantly simplified, and the present invention cancontribute significantly to a lowering in the cost of thin filmtransistor type liquid crystal display devices.

[0093] Because the present invention permits the pixel electrode to havea more simple shape than the known comb shaped electrode, the efficiencyof utilization of the light is increased. It is not necessary tosacrifice some of the aperture factor, as in conventional methods forobtaining a capacitive device which can accumulate a sufficient amountof electric charge. Replacement of the insulator for protecting the thinfilm transistors with an organic composition enables the dielectricconstant to be lower than that when an inorganic composition is used,making it possible to suppress the electric field component generated inthe vicinity of the pixel electrode in a vertical direction to theinterface of the substrate to a value smaller than the electric field ina lateral direction. This enables the liquid crystal to operate in awider region. It also contributes to an enhancement of brightness. Whenthe common electrode is used in common as the electrode for adjacentpixel electrodes, it operates in the same way as the common electrode inthe conventional active matrix type liquid crystal display device, butits structure can be simplified as compared with conventionalarrangements, and the aperture factor can be further increased.

[0094] As there is increased freedom in the selection of materials forthe liquid crystal, the orientation film and the insulator, it becomespossible to select insulating materials for the capacitive devices sothat the product of their specific resistance and the dielectricconstant is larger than that of the material of the liquid crystal.Then, one vertical scanning period in the driving signal output from thescanning wiring driving means can be set to be shorter than the timeconstant expressed by a product of the specific resistance and thedielectric constant of the insulator of the capacitive devices. Hence,the voltage variation at the pixel electrode can be reduced to asufficiently small value.

[0095] (4) The Fourth Advantage is That a Simple Thin Film TransistorStructure Having a High Aperture Factor can be Achieved, PermittingEnhancement of Brightness.

[0096] Consider the structure of a pixel including a thin filmtransistor, when comb shaped electrodes are used, as disclosed inJP-B-63-21907 (1988). There is then the problem that the aperture factordecreases significantly and the brightness is lowered. For massproduction, the necessary width of one tooth of the comb shapedelectrode is about 8 μm, with a minimum of at least 4 μm. Hence, it isimpossible to form a pixel of 0.3×0.1 mm² for a diagonal length 9.4-inch(23.9 cm) color VGA class screen with a structure having a total of 17teeth as shown in FIG. 7 of JP-B-63-21907 (1988).

[0097] The present invention permits a sufficient aperture factor to bemaintained, without losing the described advantages (1) and (2)discussed above. Instead of structures, such as comb shaped electrodes,which inevitably reduce the aperture factor, the more simple structureof the electrode permits a practical high aperture factor to beachieved.

[0098] The first aspect of the invention discussed above relates tostructures in which the common electrodes are formed on the mutuallyfacing interfaces of substrates, or the pixel electrodes are formed onthe same layer. In (JP-B-63-21907 (1988)), the directions of the signalwiring and the common electrode cross over at right angles to each otherin order to form the comb shaped electrodes. That is, the signal wiringextends in a first direction (Y direction), and the common electrodeextends in a direction perpendicular to the first direction (Xdirection). On the other hand, the present invention permits theavoidance of a complex structure, such as comb shaped electrodes byhaving the signal wiring, the pixel electrode and the common electrodeall extending in a common direction. In order to reduce the thresholdvoltage of the liquid crystal and to shorten the response time, it ispreferable to make the interval between the pixel electrode and thecommon electrode narrow. Locating the pixel electrode and the signalwiring electrode between a pair of common electrodes is also effective,and it is not necessary to use a complex structure, such as a combshaped electrode.

[0099] The second aspect of the present invention also permits thestructure to be simplified and the aperture factor increased byproviding the pixel electrode and the common electrode in differentlayers separated from each other by an insulating layer. This aspect ofthe present invention differs substantially from JP-B-63-21907 (1988) inthat the pixel electrode and the common electrode are provided inseparated layers. One advantage of this is that the region for theadditional capacitive device, which has been reduced by use of thelateral electric field system, can be further reduced. Thus, overlappingof the pixel electrode and the common electrode separated by aninsulating film becomes possible because they are in separate layers,and the load capacitance can be formed in the region of overlap. Theoverlapping parts can be used as a part of the wiring for the commonelectrodes. Hence, it is not necessary to sacrifice a part of thedisplay in order to form a capacitive device. Accordingly, the aperturefactor for the pixel can be further increased.

[0100] Depending on a design of each pixel, a plurality of capacitivedevices can be formed. Hence, the voltage holding characteristics may besignificantly improved, and deterioration of the display quality rarelyoccurs, even if severe contamination of the liquid crystal and loweringof the off-resistance of the thin film transistor are experienced.Furthermore, the insulating film formed between the pixel electrode andthe common electrode can be commonly used in common as a gate insulatingfilm of the scanning wiring (gate line) in the same layer. Therefore,there is no need to form a further film, and a process step forproviding separate layers is not necessary.

[0101] There are other advantages of the separate layers formed byinsertion of the insulating film between the pixel electrode and thecommon electrode; for example, the probability of short-circuit failuresbetween the pixel and common electrodes can be reduced significantly dueto the existence of the insulating film therebetween, and accordingly,the probability of pixel defects can be reduced.

[0102] The common electrode and/or the pixel electrode, preferably haveshapes forming a pattern which makes the aperture factor as large aspossible. The pixel electrode or the common electrode has any of anumber of flat shapes, such as a ring, a cross, a letter T, a letter π,a letter I and a ladder. By suitably combining the selected shapes, theaperture factor can be increased significantly, as compared with thecase using comb shaped electrodes.

[0103] Because the common electrode and the pixel electrode are inseparate layers with an insulating film therebetween, it becomespossible to provide electrodes having shapes which overlap each other.Hence, the present invention permits an increase in the aperture factor.When the common electrode is composed of a metallic electrode having asurface which is coated with self-oxidized film or self-nitrized film,short-circuit failure between the common electrode and the pixelelectrode can be prevented even if the two electrodes mutually overlap,so that the high aperture factor and the prevention of pixel defects arecompatible.

[0104] Embodiments of the present invention will now be described indetail.

[0105] Embodiment 1

[0106] In the first embodiment shown in FIG. 2(a) and 2(b), two glasssubstrates (not shown in FIG. 2(a) or 2(b) but as shown in FIGS. 1(a) to1(d)) were used which were polished on the surfaces and 1.1 mm thick.Between these substrates was interposed a nematic liquid crystal whichhad a positive dielectric constant anisotropy Δε of 4.5 and abirefringence Δn of 0.072 (589 nm, 20 C.°). A polyimide orientationcontrol film applied over the substrate surface was subjected to arubbing processing to produce a pretilt angle of 3.5 degrees. Therubbing directions of the upper and the lower interfaces were almostparallel and at an angle of 85 degrees (φ_(LC1)=φ_(LC2)=85°) withrespect to the direction of the applied electric field.

[0107] A gap d was formed by dispersing spherical polymer beads betweenthe substrates so that the gap became 4.5 μm when the liquid crystal wassealed. Hence, Δn·d was 0.324 μm. The resulting structure was clamped bytwo polarizing plates (not shown) (manufactured by e.g. Nitto Denko,with reference G1220DU). One of the polarizing plates had its polarizedlight transmission axis set almost parallel to the rubbing direction,i.e. φ_(P1),=85°. The polarized light transmission axis of the otherpolarizing plate was set perpendicular to the former, i.e. φ_(P2)=−5°.As a result, a normally closed characteristic was obtained.

[0108] The structure of the thin film transistor and various electrodesfor one pixel element are as shown in FIG. 2(a) and FIG. 2(b), such thatthe thin film transistor device (hatched portion in FIG. 2(a)) has apixel electrode (source electrode) 1, a signal electrode (drainelectrode) 12, and a scanning electrode (gate electrode) 10. The pixelelectrode 1 extends in a first direction (the vertical direction in FIG.2), the signal electrode 12 and the common electrodes 2 extend in thefirst direction so as to cross over a plurality of pixels (the pixelsbeing arranged vertically in FIG. 2), and the thin film transistordevice is located between the common electrodes 2. The thin filmtransistor device is shielded from light by a light shield member 18.

[0109] Signal waves having information are supplied to the signalelectrode 12, and scanning waves are supplied to the scanning electrode10 synchronously. A channel layer 16 composed of amorphous silicon (aSi)and a thin film transistor formed from an insulating protective film 15of silicon nitride (SiN) are arranged between adjacent commonelectrodes. Information signals are transmitted from the signalelectrode 12 to the pixel electrode 1 through the thin film transistor,and a voltage is generated between the common electrode 2 and the liquidcrystal 50.

[0110] In the present embodiment, the common electrodes 2 are arrangedat the facing interface side of the substrate and are enlarged in thethickness direction in the illustration in FIG. 2(b). Therefore,although the electric field direction 7 shown in FIG. 2(b) appears to beinclined relative to the horizontal, the thickness of the liquid crystallayer 5 is actually about 6 μm as compared with the width of 48 μm, sothat the inclination is very small and the supplied electric fielddirection is almost parallel to the interface of the substrate.

[0111] A capacitive device 11 was formed in a structure in which theprotruded pixel electrode 1 and the scanning wiring 10 held a gateinsulating film 13 therebetween as shown in FIG. 2(b). The electrostaticcapacitance of the capacitive device 11 was about 21 fF. Each of thelines of the scanning wiring 10 and the signal wiring 12 were connectedto a scanning wiring driving LSI, and a signal wiring driving LSIrespectively.

[0112] Electric charge accumulates in the pixel electrode 1 to about 24fF, which is the capacitance of the parallel connection of theelectrostatic capacitance between the pixel electrode 1 and the commonelectrode 2 and that of the capacitive device 11. Therefore, even if thespecific resistance of the liquid crystal 50 is 5×10¹⁰ Ωcm, the voltagevariation at the pixel electrode 1 could be suppressed, anddeterioration of the display quality was prevented.

[0113] In this embodiment, the number of the pixels were 40 (×3) ×30,and the pixel pitch was 80 μm in a lateral direction (i.e. between thecommon electrodes) and 240 μm in a vertical direction (i.e. between thescanning electrodes). A high aperture factor of e.g. 50% was obtained,with the scanning electrode 10 being 12 μm wide and the interval betweenthe adjacent scanning electrodes being 68 μm. Three stripe shaped colorfilters 17 forming respective red (R), green (G) and blue (B) filterswere provided on the substrate facing the substrate supporting the thinfilm transistors.

[0114] A transparent resin 14 was laminated on the color filters 17 forsurface flattening. The material of the transparent resin 14 ispreferably an epoxy resin. An orientation control film 4, e.g. of thepolyimide group, was applied to the transparent resin. A driving circuitwas connected to the panel.

[0115] The structure of the driving system in the present embodiment isshown in FIG. 8. A signal electrode 23 and a common electrode 31 extendto the end of the display portion. FIGS. 9 and 10 show the structure ofthe optical systems needed, FIG. 9 being for a transmission type deviceand FIG. 10 being for a reflection type device with a reflector 30.

[0116] As the present embodiment does not need any transparentelectrodes, the manufacturing process becomes simple, the productionyield increases, and the manufacturing cost can be significantlyreduced. In particular, there is no need for extremely expensivefacilities having vacuum furnaces for forming the transparentelectrodes, and there may thus be a significant reduction in investmentin the manufacturing facilities, permitting an accompanying costreduction.

[0117] The electro-optical characteristics representing the relationshipbetween the effective voltage applied to the pixels and the brightnessin the present embodiment are shown in FIG. 3(a). The contrast ratiosexceeded 150 when driven by voltages of e.g. 7 V. The difference betweenthe characterizing when the visual angle was changed laterally orvertically were significantly smaller than in conventional system (to bediscussed in comparative example 1), and the display characteristicswere not changed significantly even if the visual angle was changed. Theorientation character of the liquid crystal was preferable, and domainsof orientation defects are not generated. The aperture factor maintaineda sufficiently high value, e.g. 50% by simplifying the structure of thethin film transistor and the electrodes, and a bright display wasachieved. The average transmission factor for the whole panel was 8.4%.It should be noted that the term “brightness” is defined as thebrightness of transmission when the two polarizing plates are arrangedin parallel.

[0118] The material of the liquid crystal 50 used in the firstembodiment had a dielectric constant of 6.7 and a specific resistance of5×10¹⁰ Ωcm, and silicon nitride was used for the insulator of thecapacitive device 11 having a dielectric constant of 6.7 and a specificresistance of 5×10¹⁶ Ωcm. That means that the specific resistances ofboth the liquid crystal composition and the insulator of the capacitivedevice 11 were over 10¹⁰ Ωcm, and the product of the dielectric constantand the specific resistance of the silicon nitride, about 3×10⁴ seconds,was larger than the product of the dielectric constant and the specificresistance of the liquid crystal composition, about 0.03 seconds. Onevertical scanning period for the driving signal output from the scanningwiring driving LSI was about 16.6 ms with an ordinary liquid displaydevice, and the value satisfied the condition that the scanning periodshould be far less than about 3×10⁴ seconds. Therefore, it was possibleto derive the time constant for accumulated charge leaking from thepixel electrode 1. This facilitates the suppression of voltagevariations at the pixel electrode 1, and consequently a satisfactorydisplay quality can be obtained. The value of 5×10¹⁰ Ωcm for thespecific resistance of the liquid crystal is lower than that for theliquid crystal used for the conventional vertical electric field thinfilm transistor liquid display device, which is about 10¹² Ωcm. However,defects in the display quality were not generated.

[0119] Comparison Example

[0120] The comparison example referred to above was based on aconventional twisted nematic (TN) type of liquid crystal display device.Since this example had a transparent electrode, the structure wascomplex and the manufacturing process was long compared with the firstembodiment. The nematic liquid crystal used in the comparison examplehad a dielectric constant anisotropy Δε of positive 4.5 and abirefringence Δn of 0.072 (589 nm, 20° C.), the same as those of thefirst embodiment. The gap was set to 7.3 μm and the twist angle to 90degrees. Thus, Δn·d is 0.526 μm.

[0121] The electro-optical characteristic of this comparison example isshown in FIG. 3(b). The characteristic curves change greatly as thevisual angle changes. Near a stepped portion adjacent to the thin-filmtransistor, an orientation failure domain occurs where the liquidcrystal molecules are oriented in a direction different from that of thesurrounding portion.

[0122] Embodiment 2

[0123] In the second embodiment, the scanning electrode which had beenarranged on the substrate facing the substrate supporting the pixel inthe first embodiment was formed on the same substrate as the pixelelectrode. The rest of the structure of the second embodiment isgenerally the same as that of the first embodiment and correspondingparts are indicated by the same reference numerals. The cross section ofthe structure of the thin film transistor and the electrodes in thesecond embodiment are shown in FIG. 4. The pixel electrode 1, the signalelectrode 12 and the scanning electrode 10 were all made from aluminum,and were formed simultaneously by being deposited and etched. There isno conductive material on the other substrate. Hence, in this structure,even if the conductive material is contaminated during the manufacturingprocess, there is no possibility of upper and lower electrodes touchingeach other, and so defects due to the upper and lower electrodestouching is eliminated. There is no special restriction on the materialfor the electrodes, but it should normally be a metal having a lowelectrical resistance, and so chromium, copper, etc. are thus suitable.

[0124] Generally, the precision of alignment of photomasks issignificantly higher than that for two facing glass substrates.Therefore, variations in the alignment of the electrodes can besuppressed when all of the four electrodes are formed on only one of thesubstrates, as in the second embodiment, because alignment of theelectrodes during manufacturing can be only applied to photomasks.Therefore, the present embodiment is suitable for forming more precisepatterns in comparison with the case when the scanning electrode isformed on the other substrate.

[0125] A bright display having the same wide visual angle as the firstembodiment was obtained.

[0126] Embodiment 3

[0127] The structure of the third embodiment is generally the same asthe first embodiment 1 except as will be described below. Componentswhich correspond to components of the first embodiment are indicated bythe same reference numerals.

[0128] The structure of the thin film transistor and the variouselectrodes of the third embodiment are shown in FIGS. 5(a) and 5(b). Thesignal electrode 12 was arranged between a pair of pixel electrodes 1,and a pair of common electrodes 2 were arranged outside the aboveelectrodes. A signal wave having information is applied to the signalelectrode 12, and a scanning wave is applied to the scanning electrode10 synchronously with the signal wave. A thin film transistor comprisingamorphous silicon (a-Si) 16 and an insulating-protecting film 15 ofsilicon nitride (SiN) are arranged substantially centrally between apair of common electrodes. The same information signals are transmittedfrom the signal electrode 12 to each of two pixel electrodes 1 throughtwo thin film transistors, and the same voltage signals are applied tothe liquid crystal, each of two common electrodes at both sides havingthe same potential. With this arrangement, the distance between theelectrodes can be decreased by almost a half without making thestructure of the thin film transistor and the electrodes complex. Itthus becomes possible to apply a high electric field with the samevoltage and to decrease the driving voltage. Hence, a high response canbe achieved.

[0129] This embodiment permits the same brightness and wide visual angleto be achieved as in the first embodiment.

[0130] Embodiment 4

[0131] The structure of the fourth embodiment is generally the same asthe first embodiment except as will be described below. Components ofthe fourth embodiment which correspond to the first embodiment areindicated by the same reference numerals.

[0132]FIG. 15(a) is a partial plan view of an active matrix type liquidcrystal display device forming the fourth embodiment. FIG. 15(b) is across sectional view taken on line XVB-XVB in FIG. 15(a), and FIG. 15(c)is a cross sectional view taken on line XVC-XVC FIG. 15(a). Thecapacitive device 11 which has a structure in which the gate insulatingfilm made from silicon nitride 13 was located between the pixelelectrode 1 and the scanning wiring 10 in the first embodiment 1 hasbeen changed in this embodiment to a structure in which the liquidcrystal composition layer 50 extends between parts of the pixelelectrode 1 and the common electrode 2 which face each other, as shownin FIG. 15(c).

[0133] The fourth embodiment enables an electrostatic capacitance of thecapacitive device 11 to be connected in parallel to an electrostaticcapacitance between the pixel electrode 1 and the common electrode 2.Hence, any voltage variation at the signal wiring 10 does not affect thepixel electrode 1. Therefore, the voltage variation at the pixelelectrode 1 could be reduced, reducing variations in the display.

[0134] There is no deterioration in display quality with the activematrix type liquid crystal display device of this fourth the presentembodiment, and so the same advantages as the first embodiment can beobtained

[0135] Embodiment 5

[0136] The structure of each of the fifth to tenth embodiments aregenerally the same as the first embodiment, except as will be describedbelow. Corresponding parts are indicated by the same reference numerals.

[0137]FIGS. 16 and 17 respectively show a plan view and a crosssectional view of a unit pixel of the fifth embodiment, in which thepixel electrode 1 and the common electrode 2 are located on the sameside of the liquid crystal material and are separated by an insulatinglayer. A scanning electrode 10 and a common electrode 2 of chromium areformed on a glass substrate, and a gate insulating film 13 of siliconnitride (SiN) is formed so as to cover the above electrodes. Anamorphous film (a-Si) 16 is formed on a part of the scanning electrode10 with the gate insulating film 13 therebetween as an active layer ofthe transistor

[0138] A signal electrode 12 and a pixel electrode 1 of molybdenum areformed to overlap on a part of the pattern of the a-Si film 16, and aprotection and insulating film 15 of SiN film is formed so as to coverthe resulting structure. When the thin film transistor is operated byapplying a voltage to the scanning electrode 13 of the thin filmtransistor, a voltage is applied to the pixel electrode 1. When anelectric field is induced between the pixel electrode 1 and the commonelectrode 2 the liquid crystal molecules change their orientation in thedirection of the electric field and the light transmission changes.

[0139] In the fifth embodiment, the common electrode 2 is formed on thesame layer as the scanning electrode 10, and the pixel electrode 1 andthe signal electrode 12 are separated from the common electrode 2 by thegate insulating film 13. The device of the fifth embodiment thus differsfrom that disclosed in JP-B-63-21907 (1988), in that conventional combshaped electrodes are not used, and the pixel electrode 1 and the commonelectrode 2 overlap, with the gate insulating film 20 being disposedtherebetween. By separating the pixel electrode 1 and the signalelectrode 12 from the common electrode 2 by insulation, the designfreedom for the plan pattern of the pixel electrodes 1 and the commonelectrodes 2 is increased, and it becomes possible to increase the pixelaperture factor.

[0140] The overlapping parts of the pixel electrode 1 and the commonelectrode 2 operate as an additional capacitance which is connected inparallel to the liquid crystal capacitance, and accordingly, it becomespossible to increase the holding ability of the liquid crystal chargedvoltage. This advantage cannot be achieved by a conventional comb shapedelectrode, and the advantages are achieved only by separating the pixelelectrode 1 and the signal electrode 12 from the common electrode in aninsulating manner. As FIG. 16 reveals, it is not necessary to form acapacitive device by sacrificing a part of the display region, as in thecase when the pixel electrode and the common electrode are formed on thesame substrate, and all that is needed is to provide an overlap in apart of the wiring for leading out the common electrode outside thedisplay region.

[0141] As described above, since the design freedom for the plan patternincreases by forming the pixel electrode 1 and the signal electrode 12in a separate layer from the layer having the common electrode 2,various types and shapes of electrodes can be adopted notwithstandingthe present embodiment.

[0142] Embodiment 6

[0143]FIG. 18 shows a plan view of a unit pixel forming a sixthembodiment of the present invention in which the pixel electrode 1 andthe common electrode 2 are located in different layers separated by aninsulating layer. The cross sectional structure of the sixth embodimentis the same as that of the fifth embodiment (FIG. 17).

[0144] In the sixth embodiment, the common electrode 2 is cruciform andthe pixel electrode 1 forms a closed loop. The pixel electrode 1 and thecommon electrode 2 overlap at parts C1, C2, C3 and C4 in FIG. 18, so asto form additional capacitances. In the sixth embodiment, the distancebetween the common electrode 2 and the scanning electrode 10 can be madewide, and thus failures due to short circuits between the commonelectrode 2 and the scanning electrode 10 can be prevented. Since thepixel electrode 1 is in the form of a closed loop, normal operation canbe maintained as power is supplied to all parts of the pixel electrodeeven if a breaking occurs at an arbitrary portion of the pixelelectrode, unless a break occurs at two or more parts. That means thatthe structure of the sixth embodiment has a redundancy for breaking ofthe pixel electrode 1 and the production yield can therefore beincreased. On account of the redundancy, it becomes possible to reducethe wiring gap of the closed loop electrode 1, to make that electrode 1close to the wiring part of the scanning electrode 10 when they arearranged in different layers, thereby increasing the aperture factor.

[0145] Embodiment 7

[0146]FIG. 19 shows a plan view of a unit pixel forming a seventhembodiment of the present invention in which the pixel electrode 1 andthe common electrode 2 are located in different layers separated by aninsulating layer. The cross sectional structure of the seventhembodiment is the same as that of the fifth embodiment (FIG. 17).

[0147] In the sixth embodiment, the pixel electrode 1 is in the shape ofa letter I, and the common electrode 2 is in the form of a closed loop.In the seventh embodiment, the aperture factor can be improved as in thesixth embodiment, and the additional capacitance can be increasedbecause the extent of the overlapping of the pixel electrode 1 and thecommon electrode 2 can be increased.

[0148] Embodiment 8

[0149]FIG. 20 shows a plan view of a unit pixel forming an eighthembodiment of the present invention in which the pixel electrode 1 andthe common electrode 2 are located in different layers separated by aninsulating layer.

[0150] In the eighth embodiment, the common electrode 2 is mounted onthe substrate 3 and is separated from the scanning electrode 10 by abedding layer insulating film 33. Thus, the common electrode 2 islocated on the substrate 3 and is in a different layer separate from thelayers forming the scanning electrode 10, the pixel electrode 1 and thesignal electrode 12. Therefore, in accordance with the eighthembodiment, it becomes possible for the common electrode 2 to extend notonly parallel to the Scanning electrode 10, but also perpendicular tothe scanning electrode 10 to form a network structure. Therefore, theresistance of the common electrode 2 can be decreased, and accordingly,reductions of the wave distortion in the common voltage and preventionof smear generation can be achieved.

[0151] Embodiment 9

[0152]FIG. 21 shows a plan view of unit pixel forming a ninth embodimentof the present invention in which the pixel electrode 1 and the commonelectrode 2 are located in different layers separated by an insulatinglayer.

[0153] In the ninth embodiment, the common electrode 2 is mounted on aprotecting-insulating film 15. In accordance with the presentembodiment, the common electrode 2 is located in a different layerseparate from the layers forming all the scanning electrode 10, thepixel electrode 1, and the signal electrode 12, as in the eighthembodiment. Therefore, it is possible for the common electrode 2 toextend not only parallel to the scanning electrode 10 but alsoperpendicular direction to the scanning electrode 10 to form a networkstructure. Accordingly, the resistance of the common electrode 2 can bereduced, wave distortion in the common voltage can be decreased, andsmear generation can be prevented.

[0154] Embodiment 10

[0155]FIG. 22 shows a plan view of a unit pixel forming a tenthembodiment of the present invention in which the pixel electrode 1 andthe common electrode 2 are located in different layers separated by aninsulating layer.

[0156] In the tenth embodiment, the scanning electrode 10 and the commonelectrode 2 are made from aluminum (Al), and surfaces of thoseelectrodes 2, 10 are covered by alumina (Al₂O₃) 32, which is aself-oxidized film of aluminum. The adoption of such a structure havingdouble insulating layers decreases the possibility of failure of theinsulation between the common electrode 2 the signal electrode 12, andthe pixel electrode 1, and accordingly, pixel failure can be decreased.

[0157] Embodiment 11

[0158] The structure of the eleventh embodiment is generally the same asthe first embodiment except as will be described below. Correspondingparts are indicated by the same reference numerals.

[0159] In the eleventh embodiment, a flattening film 14 (FIG. 2(b)) madefrom a transparent polymer is laminated on the color filters 27 as anorganic insulating layer, and the surface of the film is subjected to adirect rubbing treatment without forming other films, such as anorientation control film on the flattening film 14. Epoxy resin is usedfor the material of the transparent film. The epoxy resin has twofunctions, namely that of flattening and orientation control of theliquid crystal molecules. The liquid crystal composition layer is indirect contact with the epoxy resin, and the inclination angle of theinterface is 0.5 degrees.

[0160] This structure eliminates the need to provide an orientationfilm, and makes the manufacturing easier and shorter.

[0161] Generally, in the conventional twisted nematic (TN) type, avariety of characteristics are required for the orientation film, and itwas necessary to satisfy all of those requirements. Therefore, thematerial for the orientation film was limited to a small variety ofmaterials, such as polyimide etc. The inclination angle is the mostimportant characteristic. However, as explained previously, the presentinvention does not require a large inclination angle, and thus the rangeof material selection is significantly increased.

[0162] Measurement of electro-optical characteristics of the eleventhembodiment revealed that, as in the first embodiment, the change in thecharacteristic curves was extremely small when the visual angle waschanged laterally and vertically, and the display characteristics hardlychanged. Although the inclination angle was as small as 0.5 degrees, theliquid crystal orientation was satisfactory, and no orientation failuredomain was generated.

[0163] Embodiment 12

[0164] In the twelfth embodiment, the transparent resin forming theflattening film 14 in the eleventh embodiment is changed from epoxyresin to polyimide resin. The surface of the polyimide resin is directlyrubbed so that it has both the function of flattening and the functionof orientation control of the liquid crystal molecules. The inclinationangle on the interface is 2 degrees. In comparison with otherembodiments, the display characteristics are hardly changed. The liquidcrystal orientation is satisfactory and no orientation failure domain isgenerated.

[0165] Embodiment 13

[0166] The structure of the thirteenth embodiment is the same as thefirst embodiment 1 except as will be described below. Correspondingparts are indicated by the same numerals.

[0167] The protection film 15 (FIG. 2(b)) of silicon nitride forprotecting the thin film transistor in the first embodiment is replacedwith an organic insulating layer made from epoxy resin. The surface ofthe epoxy resin is directly treated by rubbing so that it functions bothas a flattening film and an orientation control film for the liquidcrystal molecules. The inclination angle is 0.5 degrees.

[0168] Measurement of the electro-optical characteristics of thethirteenth embodiment revealed that the display characteristics werehardly changed in comparison with the first embodiment 1 . Although theinclination angle was as small as 0.5 degrees, in the eleventhembodiment, the liquid crystal orientation was satisfactory, and noorientation failure domain was generated.

[0169] Embodiment 14

[0170] In the fourteenth embodiment, the epoxy resin used as theprotection film in the thirteenth embodiment is replaced with an organicinsulating layer made from polyimide resin.

[0171] Measurement of the electro-optical characteristics in thefourteenth embodiment revealed that the display characteristics werehardly changed in comparison with the first embodiment 1 . Theinclination angle was slightly increased to 2.0 degrees in comparisonwith the thirteenth embodiment. The liquid crystal orientation wassatisfactory, and no orientation failure domain was generated.

[0172] Embodiments 15-19

[0173] The structure of the fifteenth to nineteenth embodiments are thesame as the fourteenth embodiment except as will be described below.

[0174] In the fifteenth embodiment, the directions of the long axes ofthe liquid crystal molecules on the upper and the lower interfaces (therubbing direction) are almost in parallel to each other and are set at89.5 degrees (φ_(LC1)=φ_(LC2)=89.5°) with respect to the appliedelectric field. The polarized light transmission axis of one of thepolarizing plates is set almost in parallel to the rubbing direction(φ_(P1)=89.5°) and the polarized light transmission axis of the otherpolarizing plate is set perpendicular to the first axis (φ_(P2)=−0.5°).

[0175] Similarly, in the sixteenth embodiment,φ_(LC1)=φ_(LC2)=φ_(P1)=88°, and φ_(P2)=−2.0°.

[0176] Similarly, in the seventeenth embodiment,φ_(LC1)=φ_(LC2)=φ_(P1)=75°, and φ_(P2)=−25°.

[0177] Similarly, in the eighteenth embodiment,φ_(LC1)=φ_(LC2)=φ_(P1)=45°, and φ_(P2)=−45°.

[0178] Similarly, in the nineteenth embodiment,φ_(LC1)=φ_(LC2)=φ_(P1)=30°, and φ_(P2)=−60°.

[0179] The results of the measurement of the electro-opticalcharacteristics for these embodiments are shown in a single diagram ofFIG. 7. In these embodiments, the brightness is expressed by anormalized value such that the maximum brightness in a range of appliedvoltage from zero volt to 10 volts (effective value V_(rms)) is 100% andthe minimum brightness is 0%. There is a tendency for thecharacteristics curves at the threshold to become steep as the angleφ_(LC) is increased. In order to provide a large voltage margin forhalf-tone, the angle φ_(LC) must be reduced. However, there is atendency that when the angle φ_(LC) is smaller than 45 degrees, thebrightness decreased. The optimum value of the angle φ_(LC) depends onthe number of the half-tone levels to be displayed, the requirement forbrightness, driving voltage, and whether or not the common electrode hasa voltage applied thereto. A designer can control the thresholdcharacteristics in a wide range by suitably selecting the angle φ_(LC)When considering brightness, the angle φ_(LC) is preferably at least 45degrees. An angle between 60 degrees and 89.5 degrees is morepreferable.

[0180] Measurement of the visual angle characteristics revealed that, ineach case, the characteristics curve changed very slightly when thevisual angle was changed laterally and vertically, and the displaycharacteristics were hardly changed, as in the embodiment 1.

[0181] The liquid crystal orientation was satisfactory, and noorientation failure domain was generated.

[0182] Embodiments 20-23

[0183] The greatest difference between the foregoing embodimentspreviously described and the twentieth to twenty-third embodiments isthat the dielectric constant anisotropy of the liquid crystalcomposition layer is set to be negative and the rubbing direction ischanged accordingly. A nematic liquid crystal (e.g. that known asZLI-2806 of Merck Co.) with a Δε of −4.8, and a Δn of 0.0437 (589 nm, 20C.°) is used. In the twentieth to twenty-third embodiments, thedirections of the long axes of liquid crystal molecules on the upper andthe lower interfaces (the rubbing directions, (φ_(LC1), φ_(LC2)) areapproximately parallel (φ_(LC1)=φ_(LC2)) to each other and set at anangle, φ_(LC1) exceeding zero degree and less than 45 degrees withrespect to the applied electric field. The polarized light transmissionaxis of one of the polarizing plates is set approximately parallel tothe rubbing direction (φ_(P1)) and the polarized light transmission axisof the other polarizing plate is set perpendicular to the first axis(φ_(P2)).

[0184] Thus, in the twentieth embodiment, φ_(LC1)=φ_(LC2)=φ_(P1)=1.5°,and φ_(P2)=−88.5°.

[0185] In the twenty-first embodiment, φ_(LC1)=φ_(LC2)=φ_(P1)=15°, andφ_(P2)=−75°

[0186] In the twenty-second embodiment, φ_(LC1)=φ_(LC2)=φ_(P1)=30°, andφ_(P2)=−60°

[0187] In the twenty-third embodiment, φ_(LC1)=φ_(LC2)=φ_(P1)=45°, andφ_(P2)=−45°

[0188] The gap d is set to be 6.3 μm with the liquid crystal undersealed conditions and the value of Δn·d is set to be 0.275 μm. Otherconditions, such as the structure of the thin film transistor and theelectrodes, are the same as the third embodiment.

[0189] The results of the measurement of the electro-opticalcharacteristics in these embodiments are shown in the single diagram ofFIG. 11. Unlike the case when the dielectric constant anisotropy ispositive, there is a tendency for the characteristic curves of thethreshold to become steep as the angle φ_(LC) is decreased. In order toprovide a large voltage margin for half-tone, the angle φ_(LC) must beincreased. However, there is a tendency that when the angle φ_(LC) islarger than 45 degrees, the brightness decreased. As in the case whenthe dielectric constant anisotropy is positive, the optimum value of theangle φ_(LC) depends on the number of the half-tone levels to bedisplayed, the requirement for brightness, driving voltage, and whetheror not the common electrode has a voltage applied. A designer cancontrol the threshold characteristics in a wide range by suitablyselecting the angle φ_(LC). When considering the brightness, the angleφ_(LC) is almost 45 degrees.

[0190] Measurement of the visual angle characteristics revealed that, ineach case, the characteristics curve changed very slightly when thevisual angle was changed laterally and vertically, and the displaycharacteristics were hardly changed, as in the first embodiment. Noreversions of levels in the half-tone display (8 steps) were observed ina range ±50 degrees both laterally and vertically. The liquid crystalorientation was satisfactory, and no orientation failure domain wasgenerated.

[0191] Embodiments 24 to 26

[0192] In these embodiments, the directions of the long axes of liquidcrystal molecules and the arrangement of the polarizing plates are setto be the same as the twenty-first embodiment, which produced the bestcharacteristics among the embodiments 20 to 23φ_(LC1)=φ_(LC2)=φ_(P1)=15°, and (φ_(P2)=−75°) Only the product, d·Δn, ofthe thickness of the liquid crystal composition layer d and refractiveindex anisotropy Δn was changed. In the twenty-fourth, twenty-fifth andtwenty-sixth embodiments, the thickness d of the liquid crystalcomposition layer was set to be 4.0, 4.9, and 7.2 μm, respectively.Thus, the product, d·Δn, is 0.1748, 0.2141, and 0.3146 μm, respectively.In these embodiments, the refractive index anisotropy Δn is a constantand only the thickness of the liquid crystal composition layer d ischanged. However, as well as the other type of the liquid crystaldisplay (such as the 90 degree twisted nematic type), the same resultfor the optimum brightness can be obtained even if the refractive indexanisotropy Δn is changed. Moreover, the same result can be obtained evenif the liquid crystal composition layer has a positive dielectricconstant anisotropy.

[0193] The results of the measurement in these embodiments are shown inFIG. 12. The abscissa in FIG. 12(a) indicates applied voltages, and theabscissa in FIG. 12(b) indicates d·Δn with the applied voltage being aconstant 7 Volts. As FIG. 12(b) reveals, the brightness depends stronglyon d·Δn and an optimum value exists. In order to maintain a brightnessof at least 30%, which is preferable for practical use, the value ofd·Δn should be between 0.21 and 0.36 μm; and further, if a brightnessexceeding 50% is desired, the value of d·Δn between 0.23 and 0.33 μmmust be selected. In consideration of the sealing time for the liquidcrystal, thickness control of the liquid crystal composition layer etc,and mass-production considerations, the value for d must be at least 5.0μm, and a Δn of almost 0.08 is preferable.

[0194] Embodiments 27 to 29

[0195] As the results of the twenty-fourth to twenty-sixth embodimentsreveal, the optimum value of d·Δn is between 0.21 and 0.36 μm,preferably between 0.23 and 0.33 μm. Since the thickness of the liquidcrystal composition layer which is preferable for mass-production is atleast 5.0 μm, the value of the refractive index anisotropy Δn must bealmost 0.072, preferably almost 0.066. However, the kinds of liquidcrystal compounds having such an extremely low value for Δn are verylimited, and it is very difficult for them to be compatible with othercharacteristics required for practical use.

[0196] Therefore, a new method was considered in which the values of dand Δn of the liquid crystal composition layer are set rather higherthan the optimum values, and an optically anisotropic medium having alower value for phase difference Rf than the value for d·Δof the liquidcrystal composition layer is provided. This compensates for thedeparture from the optimum value by the phase difference with the liquidcrystal composition layer. As a result, the effective phase differencewhich is generated by the combination of the liquid crystal compositionlayer and the optically anisotropic medium is in the optimum rangebetween 0.21 and 0.36 μm.

[0197] In the twenty-seventh to twenty-ninth embodiments, the structureis the same as the third embodiment except as will be described. Thethickness of the liquid crystal composition layers is set to be 5.0,5.2, and 5.5 μm, respectively. A nematic liquid crystal compositionhaving the refractive index anisotropy Δn of 0.072 (589 nm, 20 C.°) isused, so that the value for d·Δn is 0.360, 0.3744, and 0.396 μm,respectively. As the brightness and color tone are higher than thepreferable range between 0.21 to 0.36 μm, the liquid crystal cell iscolored orange. An optically anisotropic medium 28 (see FIG. 9) of anuniaxially stretched film made from polyvinyl alcohol is laminated overthe liquid crystal cell so as to compensate for the birefringent phasedifference of the liquid crystal at a low voltage driving condition (inthese embodiments, zero volts). Therefore, φ_(R) is selected to be 85degrees, being the same as φ_(LC1) (=φ_(LC2)) The phase difference Rf is0.07, 0.08, and 0.10 μm, respectively, and the value for (d·Δn−Rf) isselected to be 0.29, 0.3044, and 0.296 μm, respectively, so as to be inthe preferable range from 0.21 to 0.36 μm for the brightness and thecolor tone.

[0198] As a result, a bright display having a brightness exceeding 50%without coloring could be obtain.

[0199] Embodiment 30

[0200] In the thirtieth embodiment, the liquid crystal composition layerin the twenty-seventh embodiment is replaced with a nematic liquidcrystal composition (e.g. ZLI-4518 of Merck Co.) for which thedielectric constant anisotropy Δε is negative with a value of −2.5, andits Δn is 0.0712 (589 nm, 20 C.°). The rest of the device is the same asthe twenty-first embodiment except as will be described below. Thethickness of the liquid crystal composition layers is set to be 5.5 μm.Thus, the value for d·Δn is 0.3916 μm. An optically anisotropic mediumof an uniaxially stretched film made from polyvinyl alcohol having aphase difference Rf of 0.11 μm is laminated over the liquid crystal cellso that the value for (d·Δn−Rf) is 0.2816 μm within the preferable rangefrom 0.21 to 0.36 μm for the brightness and the color tone.

[0201] As a result, a bright display having a brightness exceeding 50%without coloring could be obtained.

[0202] Embodiment 31

[0203] The structure of the thirty-first embodiment is generally thesame as the fifteenth embodiment except as will be described below.

[0204] In the thirty-first embodiment, An of the liquid crystalcomposition layer is 0.072, and the gap is 7.0 μm. Therefore, d·Δn is0.504 μm, φ_(LC1) is 89.5 degrees, the orientation direction of theliquid crystal molecules at the upper interface and the lower interfaceare set so as to cross over perpendicularly to each other, and theangle, |φ_(LC1)−=φ_(LC2)| is 90 degrees. The polarizing plates arearranged perpendicularly to each other (|φ_(P2)−φ_(P1)|=90°), and therelationship with the orientation direction of the liquid crystalmolecules is selected so that φ_(L1)=φ_(P1), thereby providing anoptical rotatory mode. As a result, a normally open type of liquidcrystal display device is obtained.

[0205] Measurement of the electro-optical characteristics in thethirty-first embodiment revealed the result that the brightness exceeded50%, there was an extremely small difference in the characteristicscurve when the visual angle was changed laterally and vertically, andscarcely changed display characteristics could be obtained. Theorientation of the liquid crystal was satisfactory and no orientationfailure domain was generated.

[0206] Embodiments 32 and 33

[0207] The structures of the thirty-second and thirty-third embodimentsare generally the same as the first embodiment except as will bedescribed below.

[0208] In these embodiment, the polarizing plates were so arranged thatthe dark state was obtained when an electric field of low level, notzero, was applied. The value of |φ_(LC1)−φ_(P1)| was set at 5 degrees inthe thirty-second embodiment, and 15 degrees in the thirty-thirdembodiment, respectively, and the value of |φ_(P2)−φ_(P1)| was set at 90degrees for both thirty-second and thirty-third the embodiments.

[0209] A satisfactory display characteristic, for both brightness andvisual angle was obtained, as in the other embodiments. Also, the liquidcrystal orientation was satisfactory, and no orientation failure domainwas generated.

[0210] Embodiments 34 and 35

[0211] The structures of the thirty-fourth and thirty-fifth embodimentsare generally the same as the twenty-first embodiment except as will bedescribed below.

[0212] In the twenty-fourth and twenty-fifth embodiments, the polarizingplates are arranged so that the dark state is obtained when an electricfield of low level, not zero, is applied. The value |φ_(P1)−φ_(LC1)| isset at 5 degrees in the thirty-fourth embodiment and 7 degrees in thethirty-fifth embodiment, respectively, and the value of |φ_(P2)−φ_(P1)|is set as 90 degrees for both embodiments. The thickness d of the liquidcrystal composition layer is 3 μm. Therefore, d·Δn is 0.275 μm.

[0213] The results of measurement of the electro-optical characteristicsin these embodiments are shown in FIG. 13. For the thirty-fourthembodiment, the voltage causing the dark state, V_(OFF), may be 3.0Volts and the voltage causing the maximum brightness, V_(ON), was 9.2Volts. Therefore, sufficiently high contrast can be obtained if itsoperation is performed with the voltage between V_(OFF) and V_(ON).Similarly, for the thirty-fifth embodiment, V_(OFF) may be 5.0 Volts andV_(ON) may be 9.0 Volts. When it is operated with a voltage betweenV_(OFF) and V_(ON) suitable display characteristics for both brightnessand visual angle can be obtained, as in the other embodiments. Theliquid crystal orientation is satisfactory and no orientation failuredomain is generated.

[0214] Embodiment 36

[0215] The structure of the thirty-sixth embodiment is the same as thethirty-fourth embodiment except as will be described below.

[0216] In the thirty-sixth embodiment, image signals are supplied to thesignal electrode and at the same time, an alternating current at 3.0 Vis applied to the common electrode. As a result, there is a reduction ofthe voltage which is to be supplied to the signal electrode (8.3 V+Z9006.2 V). As its operation is performed with the voltage between V_(OFF)and V_(ON), satisfactory display characteristics in both brightness andvisual angle can be obtained, as in the other embodiments. The liquidcrystal orientation is satisfactory and no orientation failure domain isgenerated.

[0217] Embodiment 37

[0218] The structure of the thirty-seventh embodiment is the same as thefirst embodiment except as will be described below.

[0219] In the thirty-seventh embodiment, the polarizing plates arearranged so that the dark state is obtained when an electric field oflow level, not zero, is applied. The value of |φ_(LC1)−φ_(P1)| can beset at 45 degrees and the value of |φ_(P2)−φ_(P1)| can be set at 90degrees. Therefore, a bright state is obtained by applying a low voltageand dark state can be obtained by applying a high voltage. The resultsof measurement of the voltage dependence of the brightness in thispresent embodiment is shown with a solid line in FIG. 14. Satisfactorydisplay characteristics for both brightness and visual angle areobtained, as in the other embodiments. The contrast ratio is 35. Theliquid crystal orientation is satisfactory, and no orientation failuredomain is generated.

[0220] Embodiment 38

[0221] With the structure of the thirty-seventh embodiment, abirefringent medium (uniaxially stretched polyvinyl alcohol film) isinserted between the two polarizing plates for compensating theinterface residual phase difference. The stretched direction of the filmφ_(R) is −45 degrees, and the stretched direction crosses overperpendicularly to the transmission axis of the polarized plate. Thephase difference R_(f) is 15 nm.

[0222] As shown by a dotted line in FIG. 14, the light leakage at highvoltage is suppressed more than in the thirty-seventh embodiment, andthe contrast ratio is improved to 150. In accordance with the presentinvention, it is possible to provide:

[0223] i) firstly, a thin film transistor type liquid crystal displaydevice, having a high contrast without using a transparent electrode,which can be mass-produced with high yields at low cost by usinginexpensive manufacturing facilities;

[0224] ii) secondly, a thin film transistor type liquid crystal displaydevice having satisfactory visual angle characteristics which can easilydisplay multiple-tone images;

[0225] iii) thirdly, a thin film transistor type liquid crystal displaydevice having a large margin for the processes of the liquid crystalorientation and materials, so that the device can have a large aperturefactor, improved light transmission, and brighter images;

[0226] iv) fourthly, a thin film transistor type liquid crystal displaydevice having a large aperture factor, improved light transmission, andbrighter images by providing simple structures for the thin filmtransistor structure.

[0227] These advantages may be achieved independently or in combinationdepending on the structure of the device.

What is claimed is:
 1. A liquid crystal display device comprising: apair of substrates; a liquid crystal layer interposed between said pairof substrates; wherein one of said pair of substrates comprises: aplurality of scanning electrodes; a plurality of signal electrodesarranged so as to cross said plurality of scanning electrodes; aplurality of thin film transistors arranged in the vicinity of crossingpoints of said scanning electrodes and said signal electrodes, aplurality of common electrodes, and a plurality of pixel electrodesarranged between each of said common electrodes; wherein an electricfield is formed in said liquid crystal layer by applying a voltage tosaid pixel electrodes and said plurality of common electrodes; and theother substrate of said pair of substrates comprises: color filters, aninsulating film for flattening said color filters arranged on said colorfilters, and an orientation control film arranged on said insulatingfilm.
 2. A liquid crystal display device as claimed in claim 1, wherein:said orientation control film is arranged directly onto said insulatingfilm.
 3. A liquid crystal display device as claimed in claim 1, whereinall electrodes of said liquid crystal display device are arranged ontosaid one of said pair of substrates.